Solar cells perform better with caffeine — no, really!

Monday, 29 April, 2019

Researchers from the University of California, Los Angeles (UCLA), collaborating with China’s Solargiga Energy, have discovered that caffeine can increase the efficiency of perovskite solar cells, enhancing their thermal stability and ability to convert light to electricity.

The research, published in the journal Joule, may enable the cost-effective renewable energy technology to compete on the market with silicon solar cells.

The idea began as a joke over morning coffee, with PhD candidates Jingjing Xue and Rui Wang from the Department of Materials Science and Engineering at UCLA discussing perovskite solar cells. They wondered if coffee would boost solar cell energy and improve performance as it did for them.

The team was keen to discover if caffeine, an alkaloid compound containing molecular structures, would interact with the precursors of perovskite materials — compounds with a particular crystal structure that form the light-harvesting layer in a class of solar cells. Previous attempts to improve the thermal stability of these solar cells have included enhancing the perovskite layer by introducing compounds such as dimethyl sulfoxide, but researchers have struggled to boost the cells’ efficiency and long-term stability.

The UCLA researchers added caffeine to the perovskite layer of 40 solar cells, using infrared spectroscopy to determine if the caffeine had successfully bonded with the material. Further infrared spectroscopy tests revealed that the carbonyl groups in caffeine interacted with lead ions in the layer to create a ‘molecular lock’, which increased the minimum amount of energy required for the perovskite film to react and boosted the solar cell efficiency from 17% to over 20%. The molecular lock continued to occur when the material was heated, which could help prevent heat from breaking down the layer.

“We were surprised by the results,” Wang said. “During our first try incorporating caffeine, our perovskite solar cells already reached almost the highest efficiency we achieved in the paper.”

But while caffeine appears to significantly improve the performance of cells that utilise perovskite to absorb sunlight, the researchers do not think it will be useful for other types of solar cells. The unique molecular structure of caffeine only allows it to interact with perovskite precursors, which may give this solar cell variety an edge on the market.

Perovskite solar cells already have the advantage of being cheaper and more flexible than their silicon counterparts. They are also easier to manufacture — perovskite cells can be fabricated from solution-based precursors as opposed to solid crystal ingots. With further research, Wang believes caffeine may facilitate large-scale production of perovskite solar cells.

“Caffeine can help the perovskite achieve high crystallinity, low defects and good stability,” he explained. “This means it can potentially play a role in the scalable production of perovskite solar cells.”

To continue enhancing the efficiency and stability of the solar cells, the team plans to further investigate the chemical structure of the caffeine-incorporated perovskite material and to identify the best protective materials for perovskites.

Top image caption: A coffee mug and a perovskite solar cell that was enhanced with caffeine. Image credit: Marc Roseboro/CNSI.

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